1. Field of the Invention
The present invention is directed to force dampening devices, and more particularly to piston-cylinder aggregate dampening devices with variable dampening force fields.
2. Description of the Related Art
Piston-cylinder based force dampening devices have many industrial applications, especially in the automotive industry. For example, a commonly-assigned German Patent Application No. DE 42 12 078 A1 discloses a piston-cylinder aggregate that is designed as an end position damper. This aggregate comprises a cylindrical pressure tube, which is axially limited on one side by a bottom and on the other side by a seal guidance unit. In addition, the piston-cylinder aggregate includes a piston system, having several piston rods and flow ducts equipped with non-return valves, that subdivides the pressure tube into two steam spaces. The aggregate has a stroke area, defined as a range of the relative positions of a piston rod with respect to the pressure tube, in which no damping force exists. Only one piston of the piston system acts in the end regions of the piston rod stroke, because each piston has a non-return valve; thus, one piston produces a damping force for one movement direction and no damping force for the other flow direction, due to the non-return valve.
Another dampening piston-cylinder aggregate is shown in German Patent No. DE PS 12 51 165 that describes a shock damper with a piston on a piston rod, in which the piston consists of two piston heads. The piston works together with a rod attached to the bottom of the shock damper. The piston rod holds the piston on its outer diameter, so that an opening can be made in the center for each piston head. These center openings act, with the rod, as chokes, and the non-return valves arranged at the piston heads ensure that no damping force is produced starting from a given end position. Furthermore, the shock damper produces no damping force in a middle stroke area, because the diameter of the rod is reduced over a defined length, permitting the center openings to produce practically no damping forces. However, the damping force is produced only by the present cross-section resulting from coverage of a center opening by the rod. The piston heads, because of their non-return valves, do not participate in the damping force effect.
Yet another dampening device is shown in a commonly assigned German Patent Application No. DE 34 46 133 that discloses a vibration damper with variable damping force, in which a bypass in a damping valve device can be activated by an adjustment device in such a manner that at a given time either one of the damping valve systems in the damping valve device is producing a damping force or both damping valve systems are active.
However, none of the above or other load dampening devices provide a load-dependent dampening force which adjusts automatically to compensate for an increased load magnitude without external intervention. It would thus be desirable to provide a load dampening device for providing a dampening force of a magnitude dependent on the magnitude of the load being exerted on the device.